1. Field of the Invention
The present invention relates to a drive assembly for a track-type vehicle.
2. Background Art
One drive assembly used in the related art for a track-type vehicle is provided with bogie (or rocking type) track rollers for improving the riding comfortableness at the running time of the vehicle (as referred to in FIG. 1 to FIG. 5 of U.S. Pat. No. 6,607,256. FIG. 7 is a side elevation of a bulldozer as the track-type vehicle disclosed in U.S. Pat. No. 6,607,256, and FIG. 8 is a side elevation of a drive assembly of the track-type vehicle.
In FIGS. 7 and 8, the track-type vehicle is provided with a drive assembly having crawlers 6 below the right-hand and left-hand sides of its body. The drive assembly is provided with a pair of track frames 1 and each track frame 1 is arranged along a longitudinal direction of the vehicle. Further, the an idler 2 and a sprocket 3 are disposed at a front end portion and a rear portion of the track frame 1, respectively. Below the track frame 1 and at a position closest to the idler 2, there are provided a single track roller unit 40 having an arm 41 whose proximal end portion is attached to the track frame 1 via a pin 42 so as to vertically rock and a track roller 43 rotatably mounted on a distal end portion of the arm 41.
Below the track frame 1 and between the single track roller unit 40 and the sprocket 3, there is provided a double track roller unit 10 comprising a first arm 31 whose proximal end portion is attached to the track frame 1 via a first pin 33 so as to vertically rock, a second arm 32 whose central portion is attached to a distal end portion of the first arm 31 via a second pin 35 so as to vertically rock and a pair of longitudinal track rollers 11 and 12 which are rotatably mounted at two end portions of the second arm 32.
On an abutment portion between the distal end portion of the arm 41 of the single track roller unit 40 and a lower portion of the track frame 1 and at an abutment portion between an upper portion of the distal end portion of the first arm 31 of the double track roller unit 10 and the lower portion of the track frame 1, respectively, there are mounted elastic members 44a and 44b, and 34a and 34b, which have predetermined moduli of elasticity. Loads to be applied to the track roller 43 of the single track roller unit 40 and to the track rollers 11 and 12 of the double track roller unit 10 are respectively borne by upper and lower elastic members 44a and 44b, and 34a and 34b through the arm 41, and the second arm 32 and the first arm 31.
Here in the drive assembly for the track-type vehicle according to the related art described above, a position for the track roller to support a vehicle load is located at the elastic members 44a and 44b in the single track roller unit 40 and is substantially identical to a position of the track roller 43. In the double track roller unit 10, on the contrary, a support position is located at that of the elastic members 34a and 34b and is substantially identical to a center position between the paired front and rear track rollers 11 and 12.
Usually in the track-type drive assembly, a distance between the track rollers on longitudinally outermost sides for supporting the vehicle load is called an “effective ground contact length”, by which a stability at a vehicle running time is retained. Therefore, the effective ground contact length of a running device of the related art is a distance between the track roller 43 of the single track roller unit 40, as located closest to the idler 2, and a center position between the front and rear track rollers 11 and 12 of the double track roller unit 10 located closest to the sprocket 3, so that the length is substantially shorter than the distance between the outermost track rollers.
Merely in order to enlarge the effective ground contact length, generally speaking, it is naturally sufficient to elongate the distance (which will be called a “tumbler distance”, as usually called so) between the idler and the sprocket thereby to elongate the distance between the outermost track rollers accordingly. With this modification, however, the track frame 1 is large-sized, and the running length of a crawler 6 is enlarged to raise a cost. Therefore, it has been desired to make a best use of a space within the tumbler distance thereby to make the effective ground contact length as large as possible without elongating the tumbler distance.